The alt.usage.english FAQ

"a"/"an" before abbreviations

(Foreigners' Faqs)

"A" is used before words beginning with consonants; "an", before
words beginning with vowels. This is determined by sound, not
spelling ("a history", "an hour", "a unit", "a European", "a one").
Formerly, "an" was usual before unaccented syllables beginning with
"h" ("an historian", "an hotel"); these are "now obsolescent" in
British English (Collins English Dictionary), although "an
historian" is retained in more dialects than "an hotel".
Before abbreviations, the choice of "a"/"an" depends on how
the abbreviation is pronounced: "a NATO spokesman" (because "NATO"
is pronounced /'neItoU/); "an NBC spokesman" (because "NBC" is
pronounced /Enbi:'si:/) "a NY spokesman" (because "NY" is read as
"New York (state)").
A problem: how can a foreigner *tell* whether a particular
abbreviation is pronounced as a word or not? Two non-foolproof
guidelines:
(1) It's more likely to be an acronym if it *looks* as if it could
be an English word. "NATO" and "scuba" do; "UCLA" and "NAACP"
don't.
(2) It's more likely to be an acronym if it's a *long* sequence of
letters. "US" is short; "EBCDIC" is too bloody long to say as
"E-B-C-D-I-C". (But of course, abbreviations that can be broken
down into groups, like "TCP/IP" and "AFL-CIO", are spelled out
because the groups are short enough.)
Is it "a FAQ" or "an FAQ"? These days, probably the former,
although some of us do say "an F-A-Q".